INVESTIGATION ON DC/AC MICROGRID STABILITY FOR INPUT SOURCE DISTURBANCE

Abstract

The idea of a microgrid is to integrate a selected number of DGs with proper control strategy and loads and should have a point of common coupling (PCC) point for connection/disconnection with the grid. While remain connected to the grid, it is called working in a grid-connected mode, and the inverter functions in constant current control mode or active/reactive (P/Q) mode. Due to certain faults or interruptions, the PCC point is opened, and the microgrid functions autonomously; hence it is referred to as working under autonomous mode or islanded mode. In autonomous mode, the microgrid operates in constant voltage control mode or V/f control mode. In a microgrid, the selected number of distributed sources are operated in parallel to each other and interfaced through power electronic devices resulting in a large number of power electronic devices being used. Hence the modeling of system components is studied, which is used for stability analysis and designing of filters and droop gains, and other significant parameters. Moreover, the control strategy is developed based on the system model, and a simulation study is performed for verification of controller design in the time domain. This thesis presents systematic modelling approach for grid-connected as well as autonomous mode DC/AC hybrid microgrid (μG) using Battery Energy Storage System (BESS) and distributed energy resource (DER). The hybrid DC/AC system is a consolidation of a Permanent Magnet Synchronous Generator (PMSG) based dc to dc converter driven wind turbine, Photovoltaic array, and Battery Energy Storage System (BESS) at a common dc-bus. The BESS system ensures continuous power support and takes care of the infrequent nature of photovoltaic and wind. The decentralized control is applied in the proposed grid-interactive configuration during variable wind speed and solar irradiance source disturbance to achieve three goals, namely, regulating the dc-bus voltage, extracting maximum power from distributed generation, and supplying energy-efficient power to the grid. Distributed energy resource (DER) unit is represented by a DC/AC voltage source interfaced through power electronic devices for supplying suitable loads. Modeling is implemented for these elements, and then a control strategy is developed for the proper regulation of output power in grid-connected mode and regulation of voltage and frequency in autonomous mode. Each subsystem is modeled separately and then all are interfaced. Load sharing is carried out among distributed generations in autonomous mode.